Over the past twenty years researchers have created a nearly uncountable number of MEMS sensors and actuated structures using the tools of microfabrication. The scientific and engineering literature contains many examples of high-performance devices. There are, however, various barriers to manufacturing successful designs. While the fabrication methods MEMS designers have inherited from the microelectronics industry permit the construction of remarkably detailed and complex moving structures, fabrication-induced stresses and resulting fractures often result in a very poor yield.
One type of device which can be produced using micro-electro-mechanical systems (MEMS) technology is a microphone. Microphones can be omnidirectional, meaning that the microphone is responsive to the dynamically changing pressure incident on the diaphragm from all directions, or directional, meaning that the microphone is selectively sensitive to sound waves of particular propagation axes.
The advantages of MEMS microphones over electret condenser microphones (ECMs) are their size, performance, sound quality and suitability for mass production. The advantages of a microphone according to this design are its small size, low internal noise, low power consumption and high directionality. All of these features have been demonstrated during evaluation of a prototype and confirm its superior performance over existing commercially available MEMS directional microphones [7].
A particular type of directional microphone provides a “counterbalanced” diaphragm in which two adjacent portions, separated by a central pivot axis, are exposed to sound. The pivoting diaphragm is selectively responsive to acoustic waves depending on the inclination of the acoustic wave with respect to the plane of the diaphragm and pivot axis. This type of directional microphone is inspired by the acoustic sensory organ of Ormia ochracea [42].
The pivoting diaphragm itself requires a rotating support, which concentrates stress at the pivoting support. The basic design of such a directional microphone is shown in FIGS. 1A and 1B. These MEMS sensors are used, for example, to construct biomimetic directional microphones for hearing aid applications [1-13].
The fabrication of parallel plate electrodes is a common way to achieve capacitive sensing for microphones and other planar displacement sensors, and may be accomplished by depositing a sacrificial oxide film (typically having a thickness in the range of 5 to 10 microns) and then depositing a conductive material, such as silicon or metal, on top of the oxide. The space between the conducting electrode and the moving diaphragm may then be opened through a release etch, creating a diaphragm membrane which floats over a backplate, supported by a pair of opposed hinges. A disadvantage of employing parallel plate sensing is that the air between the diaphragm and the fixed electrode is squeezed as the diaphragm moves and must flow in the plane of the diaphragm. The viscous force due to the flow of air depends on the third power of the distance between the diaphragm and the stationary electrode, and can be a dominant source of damping in the system [19-26]. It is, of course, desirable to have this gap be as small as feasible to increase the amount of capacitance and the overall sensitivity of the device. While holes in the electrode can be designed to reduce the damping, it can be very difficult to design a system that has both high sensitivity and low damping, which is desired in a device that must move quickly.
An additional substantial difficulty with the use of parallel plate electrodes is that the bias voltage that is required between the electrode and the diaphragm results in an attractive force that compels the two surfaces to reduce the size of the gap. If the mechanical restoring force provided by the hinge is not sufficiently high, the use of too large of a bias voltage will cause the gap between the diaphragm and electrode to collapse completely, with the rear of the diaphragm contacting the backplate.
The use of interdigitated fingers at the periphery of the diaphragm can overcome many of the disadvantages of parallel plate electrodes, and can be fabricated in the same steps and mask as used to create the slits around the diaphragm.
While the use of interdigitated fingers solves many difficult problems inherent in parallel plate MEMS devices, there are practical difficulties that have greatly impeded their use. The main difficulties are that the capacitance that can be achieved is lower than desired, due to the small surface area of the interdigitated fingers and therefore small change in charge to the movement of the fingers, and that fabrication-induced stresses can result in cracks in the polysilicon film resulting in poor fabrication yield. As a result of these difficulties, nearly all of the 2 billion microphones produced each year employ parallel plate electrodes to achieve capacitive sensing.
Another example of a pivoting planar microstructure MEMS device technology is a so-called micromirror, which is an actuator driven device [14-18]. It is important that these devices be able to move as quickly as possible (thus implying maintaining a low inertia), and in many designs, they are designed to rotate about a hinge axis in a similar manner to the concept shown in FIGS. 1A and 1B. Reducing the inertia along with the stiffness of the supporting hinge also reduces the voltage (and power) needed for capacitive actuation. These systems are thus subject to the same stress concentration at the pivot as the concept of FIGS. 1A and 1B.
In an interdigitated electrode finger structure, a series of N fingers having length 1 extend in the plane of the diaphragm into a corresponding set of fingers held in fixed position, overlapping by a distance h and separated by a gap d. The diaphragm is free to move normal to its plane in the x direction, and thus the respective sets of fingers have a displacement related to the amount of movement. It is noted that the capacitance of the structures is generally the sum of the respective capacitances of the fingers, and therefore if there is variation, the individual values may be calculated and summed. The total capacitance C of a microphone structure using the interdigitation technique may therefore be roughly estimated by:
  C  =                    ɛ        ⁢                                  ⁢                  (                      h            -            x                    )                    d        ⁢    l    ⁢                  ⁢    2    ⁢    N  
If a bias voltage Vb is then applied between diaphragm and back plate, for example to permit sensing of the displacement, the resulting electrostatic force f (for small x, neglecting fringing effects) will be:
  f  =                    d        dx            ⁢              (                              1            2                    ⁢                                    ɛ              ⁡                              (                                  h                  -                  x                                )                                      d                    ⁢          l          ⁢                                          ⁢          2          ⁢                      NV            b            2                          )              =                  -                  ɛ          d                    ⁢              lNV        b        2            
The bias voltage Vb does not reduce the stability of the diaphragm's motion in the x direction; a high bias voltage Vb may be used without a need to increase diaphragm stiffness, resulting in increased microphone sensitivity (for equal capacitance) without the diaphragm collapse problems. The applied static voltage results in an attractive force that acts to bring the moving sensing electrode toward the fixed electrode. In the case of the present comb-sense microphone, this attractive force acts to bring the microphone diaphragm toward its neutral position (i.e., x=0), in line with the fixed fingers. As a result, the bias voltage tends to stabilize the diaphragm rather than lead to instability. As long as the fingers are designed so that they themselves will resist collapsing toward each other, the diaphragm's compliance does not need to be adjusted to avoid collapse against the fixed electrodes. For example, the interdigital fingers may be provided on opposing sides of the diaphragm structure, so that the forces tending to displace it with respect to the finger gap balance each other. This means that the diaphragm may be designed to be highly compliant and thus very responsive to sound.
The following U.S. patents are expressly incorporated herein in their entirety: U.S. Pat. Nos. 7,041,225; 7,318,349; 7,520,173; 7,724,417; 7,808,640; 7,825,484; 7,903,835; 8,129,803; 8,165,323; 7,542,580; 6,963,653; 7,471,798; 7,545,945; 7,826,629; 7,992,283; 8,073,167; 7,146,016; 7,146,014; and 6,788,796.